More smog problems

[Bill]

My 90 is not passing smog. NO is about 800 needs to be less than 541. One problem we are seeing is the out put from the left side O2 sensor pulses negative when warmed up. The car is equipped with exempted SSW headers. Has anyone experienced anything similar?

[erikszr1]

sounds like that o2 is either stuck, meaning bad or possibly a vacumm leak...just somthing i thought. hope this helps

[Bill]

I should have mentioned in the first post, the offending O2 sensor had been changed, no change. Still investigating a possible vacuum problem. Still pondering the O2 sensor outputting a negative voltage.

[SoCalZR1]

My 90 is not passing smog. NO is about 800 needs to be less than 541. One problem we are seeing is the out put from the left side O2 sensor pulses negative when warmed up. The car is equipped with exempted SSW headers. Has anyone experienced anything similar?

My 90 just passed. Long story short NO was only a problem when the ambient temp was high. When I passed the other day NO was in mid three hundreds. It had failed the week before when the ambient temp was high, around 94. The tester did not have a fan running as they should above 75 degrees. NO on that attempt was over 700. Do you know what the temp was. I am also curious what RPMs were you running at 15 and 25 MPH. Did it fail at both speeds?

Mike